The present invention relates to a semiconductor photodetector, and more particularly to a planar type heterojunction avalanche photodetector having a guard ring effect and its manufacturing method.
Attempts are being made to develop avalanche photodiodes (APDs) using In.sub.0.53 Ga.sub.0.47 As as light detectors for use in optical communications in the 1.0 to 1.6 micron wavelength region, where the transmission loss of optical fibers is relatively small. Since In.sub.0.53 Ga.sub.0.47 As is lattice-matched to InP, and permits the formation of a heterojunction, it is possible to realize a photodetector which uses InGaAs and InP as a light absorbing layer and an avalanche gain layer, respectively. Such a photodetector is designed to achieve avalanche gain by conveying either the electron or the hole carrier which is generated in the InGaAs layer by optical excitation to the InP avalanche gain layer, resulting in reduction in dark current and excess noise and accordingly in a higher receiver sensitivity. K. Nishida et al. proposed this idea in Appl. Phys. Lett., Vol. 35, No. 3, pp. 251-253 (1979). FIG. 1 illustrates the structure proposed by Nishida et al., wherein a p.sup.+ -n junction is formed by providing a p-type conductivity region 5 after successively growing on a substrate 1 an n-InP buffer layer 2, an n.sup.- -In.sub.0.53 Ga.sub.0.47 As layer 3, an n-InP layer 4 and an n.sup.- -InP layer 4'. The p.sup.+ -n junction is sometimes called a one-sided abrupt or a one-sided step junction wherein acceptor impurity concentration is much greater than donor impurity concentration particularly away from the abrupt or step p-n junctions. In the abrupt or step junction, the impurity concentration in a semiconductor changes abruptly from acceptor impurities to donor impurities. Therefore, when a reverse bias voltage is applied to the p-n junction, the conductivity region with lower carrier concentration is selectively depleted. Further in FIG. 1, reference numerals 6, 7 and 8 respectively represent a surface protecting film which also provides anti-reflection, a p-electrode and an n-electrode. Under the application of a reverse bias voltage which is enough for the depleted region to reach the InGaAs layer 3, the InGaAs layer 3 having a smaller bandgap absorbs light and only the positive hole carrier generated therein is conveyed to the p.sup.+ -n junction formed in the InP layer 4 having a larger bandgap to achieve avalanche gain. A voltage breakdown occurs in the InP layer 4 with the generation of a tunnel current from the InGaAs layer 3 suppressed, achieving a low dark current photodetector.
In the structure of FIG. 1, however, the peripheral part 5a of the selectively formed p.sup.+ -n junction has its center of curvature within the p-type conductivity region 5 (this being referred to as having a "positive curvature") and, when a reverse bias voltage is applied to the p-n junction, high electric field concentrates in this "positive curvature" part 5a, resulting in a voltage breakdown thereat at a voltage lower than in the planar part 5b of the p-n junction (a so-called edge breakdown). This edge breakdown is particularly conspicuous when the carrier concentration of the InGaAs layer 3 is much lower than that of the InP layer 4. This fact means that no sufficient carrier avalanche gain is achieved in the planar part 5b corresponding to the light receiving region.
T. Shirai et al. proposed the structure illustrated in FIG. 2 to suppress this edge breakdown (Electron. Lett., Vol. 19, No. 14, pp. 534-535, 1983). In this structure, a p-type conductivity region 5' (a so-called guard ring), wherein a graded p-n junction whose breakdown voltage is higher than in a p.sup.+ -n junction, or another p-n junction which can be approximated to the graded type is provided in such a position in the peripheral part of the p.sup.+ -n junction that the depth from the surface of the graded or similar p-n junction is substantially equal to that of the p.sup.+ -n junction. The graded p-n junction here means a p-n junction wherein concentrations of donor and acceptor change substantially linearly in the vicinity of the p-n junction. Accordingly, a junction whose depletion region, when a reverse bias voltage is applied to the graded p-n junction, is extended approximately equally toward the p-type and n-type conductivity regions. Even with the structure of FIG. 2, however, it is difficult to realize high avalanche gain while preventing the edge breakdown. The reason will be explained below. When a reverse bias voltage is applied to the p-n junction, while the depletion region of a p.sup.+ -n junction grows mainly toward the n-type conductivity region, that in a graded p-n junction grows on both sides toward both the p-type and n-type conductivity regions. Therefore, the peripheral depletion region ends 5a of a p.sup.+ -n junction have positive curvatures, as indicated by the depletion distribution 5d marked with oblique lines in FIG. 3 and accordingly, like in the aforementioned case of FIG. 1, an edge breakdown is apt to ultimately occur in this positive curvature part 5a.
Then a structure wherein the positive curvature parts 5a in FIG. 1 are completely enclosed, may be effective. Such a structure is well known to be effective for an APD composed of a single semiconductor, such as an Si-APD or a Ge-APD (see, for example, I. Hino et al., "Ge APD Characteristics for Optical Communications", NEC RESEARCH & DEVELOPMENT, No. 67, October 1982, pp. 67-72.). The inventors tried to apply such a structure to a heterojunction APD using compound semiconductors, and however found it difficult to achieve uniform avalanche gain with a high reproducibility, with its edge breakdown sufficiently restrained. FIG. 4 illustrates the heterojunction APD that the inventors tried. In this structure, The junction position of a guard ring 5' comes closer to the hetero-interface between an InGaAs layer 3 and an InP layer 4 than to a p.sup.+ -n junction 5b, so that the electric field strength in the InGaAs layer is greater in the region beneath the guard ring part than in the region beneath the p.sup.+ -n junction part. Therefore, there emerges in the guard ring part an effect of the voltage breakdown in the InGaAs layer. This effect is the strongest in the positive curvature parts 5'a of the guard ring, and is due to the fact that voltage breakdown occurs in the positive curvature parts 5'a on the periphery of the guard ring before a breakdown takes place in the p.sup.+ -n junction part.